Marriage within one's own tribe or group as required by custom or law WordNet 3.0 & Open English WordNet

Endogamy is the cultural practice of mating—usually in the form of marriage—within a specific social group, religious denomination, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close interpersonal relationships. Wikipedia

The practice of marrying within a specific ethnic group, class, or social group Wikipedia Disambiguation

Marriage within a particular group in accordance with custom or law. OmegaWiki

The practice of marrying or requiring to marry within one's own ethnic, religious, or social group. Wiktionary

The practice of marrying within one's own social group. Wiktionary (translation)

Marrying inside one’s own ethnic, religious, or social group. Wiktionary

Marriage between people belonging to different groups, such as different racial, ethnic, or religious groups; mixed marriage. Wiktionary